The present invention relates to improved disposable, wet packaged or premoistened paper products, which have superior wet tensile strength at acidic pH and yet which have substantially reduced wet tensile strength at neutral or alkaline pH for ready disposal. These paper products, which are generally used for skin cleansing, are known commercially as towelettes, wet wipes or fem-wipes, and are formed from paper or non-woven fibrous webs which are treated with an improved polymeric binder which gives substantially higher tensile strength compared to prior art binder treated products when stored in an acidic wetting medium and during usage yet which exhibited substantially reduced strength when disposed in neutral or alkaline pH medium. The invention also related to a method of preparing an improved strengthened premoistened paper product through treatment of fibrous web material with an improved polymer binder prepared by the reaction of glyoxal and polyvinyl alcohol prior to drying and then wetting of the treated paper product with an acidic solution.
Current premoistened paper products are prepared by treating paper or fibrous webs, which have been prepared by conventional paper manufacturing processes, with various polymer binders to impart a degree of wet strength to the web when packaged in contact with an acid aqueous medium. These wet tissues or cleansing products are normally stored in sealed packages until used, thus requiring that they maintain their strength during storage and use and yet be readily disposable when flushed in plain water without clogging of typical plumbing and toilet equipment.
Various binders and processes for the manufacture of premoistened paper products have been proposed in the prior art. Thus, for example, U.S. Pat. No. 4,117,187 to James W. Adams discloses a premoistened wipe prepared through use of an acid-insoluble, alkali-soluble polymeric polycarboxylic acid and functional derivative thereof wherein the acid is dissolved in water and enough alkali is added to substantially neutralize all acidic groups prior to application to the fiber web. The binder saturated web is dried and then immersed in a low pH medium to effect an association reaction between the carboxylic group and the cellulose of the web, presumably through hydrogen bonding. These hydrogen bonds provide a reasonably strong linkage in acidic medium and yet will break-up when the wipe is immersed in a sufficiently high pH liquid medium.
Canadian Pat. No. 948,802 to David V. Duchane discloses a non-woven fibric wrapper from rayon fabric formed by conventional wet lay or dry lay processes which is first treated with a cold water soluble polyvinyl alcohol binder, e.g. by spraying, and then oversprayed with a solution of gelling or insolubilizing agent such as borax, to crosslink at least the surface area of the polymer binder before heating to dry to give a water resistant web. Thus Duchane utilizes the complexing reaction between borax and polyvinyl alcohol under heating to give wet strength in an acidic moist environment and yet disposability through dissociation of the polyvinyl alcohol-borax complex upon soaking in excess neutral water. This complexing reaction between borax and polyvinyl alcohol is similar to complexing between boric acid and polyvinyl alcohol as disclosed in R. F. Nickerson in Appl. Polymer Science 15, 111, (1971).
Canadian Pat. No. 1,109,758 to Gerald Miller similarly discloses the use of the reaction of a polyvinyl alcohol binder and boric acid or soluble salts thereof to form complexes having di-diols cross-linkages which are of a gel nature for binding the web. As disclosed in the above mentioned article of R. F. Nickerson, the borate ion is the effective crosslinking agent for the polyvinyl alcohol, and boric acid, at concentrations greater than 0.03 M as in Miller's patent, contributes sufficient amounts of borate ions to provide a gel type complex for binding the web as in the above mentioned Canadian patent of Duchane.
Canadian Pat. No. 1,104,026 to Daniel et al. discloses the use of a dried emulsion of a vinyl acetate-ethylene copolymer binder containing cold water soluble polyvinyl alcohol as a protective colloid with boric acid, which is used to temporarily insolubilize the binder. The polyvinyl acetate or ethylene-vinyl acetate copolymers have no known reaction with boric acid. It is the presence of the polyvinyl alcohol in the emulsion as protective colloid which forms a complex with the boron ions of the boric acid in the same manner as taught in the above patents of Duchane and Miller.
The instant invention has, through the use of a unique glyoxalated polyvinyl alcohol copolymer binder for pre-moistened paper products, achieved a substantially higher wet strength wiper when wetted in acidic medium than that achieved with prior art binders, as shown in comparison examples, and yet which maintains equal or better disposability in neutral pH water.